1. Field of the Invention
The present invention relates to a dichroic prism used for color synthesis performed by a liquid crystal projector or the like and a method for manufacturing the same.
2. Related Background Art
In a full color liquid crystal projector, white light from a light source is resolved into three primary colors of light, which are red light, blue light, and green light, by a color resolving cross prism. Each of the three primary colors of light resolved by the color resolving cross prism are made incident on a liquid crystal display element corresponding to each color. The light made incident on the liquid crystal display element is modulated based on image information. The three lights outputted from the liquid crystal display element corresponding to the respective colors are synthesized into one light by the color synthesizing cross prism and projected on a screen.
The cross prism for synthesizing the three primary colors of light is a dichroic prism on which dichroic films having different reflection characteristics are formed on two diagonal planes, respectively. Such a dichroic prism consists of four right angle prisms. Each right angle prism has a triangle columnar shape whose cross section is a right isosceles triangle, having a first side face and a second side face in a mutually perpendicular relation.
Conventionally, the dichroic prism is manufactured in the following way. First, using two right angle prisms of four right angle prisms, the first side face of one of two prisms is bonded to the second side face of the other prism to manufacture a bonded prism. Moreover, the remaining two right angle prisms are similarly bonded to each other to manufacture another bonded prism. Then, these bonded prisms are bonded to each other to manufacture a dichroic prism.
The minimum angle formed by the first side face and the second side face in each of the four right angle prisms of the dichroic prism is substantially a right angle. However, in some cases, the minimum angle is slightly larger than a right angle or slightly smaller than a right angle. Moreover, the two bonded prisms are conventionally bonded to each other without using a special jig but by hand, and adjusted depending on the human eye. Consequently, when the dichroic prism is manufactured by bonding four right angle prisms, in some cases, a gap may be created at the center thereof.
When a gap is created at the center of the dichroic prism, a light made incident thereon is reflected at a different part because of the gap. Therefore, a double-line is generated on a screen on which a reflected light is projected. Moreover, for the fact that the minimum angle formed by the first side face and the second side face of the right angle prism is not a right angle, image sizes differ for each color light made incident on the dichroic prism.
As a method of reducing the gap at the center of the dichroic prism, a conventionally known technique provides a method for bonding right angle prisms by making a notch on part of the right angle prism and applying a guide to the notch (For example, see Japanese Patent Laid Open No. Hei 10-39119). In this case, since the guide is used, the gap portion can be reduced, compared with the case where right angle prisms are bonded by eye-adjustment. If the gap at the center is reduced, the condition that generates a double-line on the screen is improved.
However, the right angle prism cannot be freed from adverse effects due to the condition that the minimum angle formed by the first side face and the second side face thereof is not a right angle, that is, it cannot be freed from non-uniformity of the screen size caused by right angle precision. In order to prevent the degradation in image quality due to the right angle precision, the right angle precision is required to be kept at an extremely high level. However, if the precision is kept at an extremely high level, costs of polishing the right angle prism and the like are increased.
The present invention is accomplished in view of the above-described problems, and a major object of the present invention is to provide a dichroic prism capable of improving image quality while suppressing the cost, and a method for manufacturing the same.
According to one aspect of the present invention, provided is a dichroic prism comprising a first right angle prism, a second right angle prism, a third right angle prism, and a fourth right angle prism, each having a first and second side faces substantially perpendicular to each other, wherein the dichroic prism is a columnar body having a square shape in lateral cross section, the columnar body including: a first bonded face formed by bonding the first side face of the first right angle prism and the second side face of the second right angle prism; a second bonded face formed by bonding the first side face of the second right angle prism and the second side face of the third right angle prism; a third bonded face formed by bonding the first side face of the third right angle prism and the second side face of the fourth right angle prism; and a fourth bonded face by bonding the first side face of the fourth right angle prism and the second side face of the first right angle prism. The first bonded face and the third bonded face have dichroic films for first color light, which reflect the first color light and transmit there through second color light different in color from the first color light. Also, the second bonded face and the fourth bonded face have dichroic films for the second color light, which reflect the second color light and transmit the first color light therethrough. Further, the second bonded face and the fourth bonded face are arranged on the same plane, and the third bonded face is deviated from the first bonded face by a predetermined distance.
The distance is one, in which an image formed of the first color light having predetermined reference image information coincides with an image corresponding to the reference image information, the first color light being allowed to be incident onto the first and third bonded faces and being reflected thereon.
When the third bonded face is deviated with respect to the first bonded face by this distance, obtained is an optical quality of the image formed by the first color light having image information which is made incident on and reflected by the dichroic prism according to the present invention.
In addition, according to another aspect of the present invention, a method for manufacturing the dichroic prism described above is provided. The method for manufacturing the dichroic prism of the present invention includes: a first step of preparing a first right angle prism, a second right angle prism, a third right angle prism, and a fourth right angle prism, each having first and second side faces substantially perpendicular to each other; a second step of forming a dichroic film for first color light on the first side face of the first right angle prism, the dichroic film for the first color light reflecting the first color light and transmitting therethrough second color light different in color from the first color light, and forming a dichroic film for the second color light on the second side face of the first angle prism, the dichroic film for the second color light reflecting the second color light and transmitting the first color light therethrough; a third step of forming the dichroic film for the second color light on the first side face of the second right angle prism; a fourth step of forming the dichroic film for the first color light on the second side face of the fourth right angle prism; a fifth step of arranging the second side face of the first right angle prism and the first side face of the second right angle prism on the same plane, and of bonding the first side face of the first right angle prism and the second side face of the second right angle prism, thus manufacturing a first bonded prism in which a first bonded face having the dichroic film for the first color light is formed; a sixth step of arranging the second side face of the third right angle prism and the first side face of the fourth right angle prism on the same plane, and of bonding the first side face of the third right angle prism and the second side face of the fourth right angle prism, thus manufacturing a second bonded prism in which a third bonded face having the dichroic film for the first color light is formed; a seventh step of constituting a pre-dichroic prism by allowing a plane, which is formed of the second side face of the first right angle prism and the first side face of the second right angle prism in the first bonded prism, and a plane, which is formed of the second side face of the third right angle prism and the first side face of the fourth right angle prism in the second bonded prism, to face to each other and to contact each other; an eighth step of adjusting a position of the second bonded prim with respect to the first bonded prism such that an image formed of the first color light having predetermined reference image information coincides with an image corresponding to the reference image information, the first color light being allowed to be incident onto the first and third bonded faces in the pre-dichroic prism and being reflected thereon, thus deviating the first bonded face and the third bonded face from each other; and a ninth step of bonding the first bonded prism and the second bonded prism, thus forming the second bonded face and the fourth bonded face, each having the dichroic film for the second color light.
According to the method for manufacturing the dichroic prism of the present invention, the first bonded prism is manufactured so that the second side face of the first right angle prism and the first side face of the second right angle prism are arranged on the same plane. In addition, the second bonded prism is manufactured so that the second side face of the third right angle prism and the first side face of the fourth right angle prism are arranged on the same plane. Accordingly, when the first bonded prism and the second bonded prism are brought into contact to be formed in a columnar body having a square shape in lateral cross section, the angle formed by the second bonded face and the fourth bonded face, each having the dichroic film for the second color light, becomes precisely 180 degrees. Moreover, when the angles formed by the first side faces and the second side faces of each of the first to fourth right angle prisms are not 90 degrees, the error gives a adverse effect on the angle formed by the first bonded face and the third bonded face, each having a dichroic film for the first color light.
Meanwhile, the first bonded prism and the second bonded prism are adhered to form a pre-dichroic prism. In this state, the first color light is made incident on and reflected by the first bonded face and the third bonded face, thereby forming an image on the screen by the first color light reflected. Then, while observing the image, the position of the second bonded prism is adjusted so that the first bonded face and the third bonded face maybe deviated, in order to improve the image quality. Therefore, an optimal image quality can be obtained even when the angle formed by the first bonded face and the third bonded face is not 180 degrees. Thus, the image quality can be improved without depending on the right angle precision of the first to fourth right angle prisms, thereby not requiring sophisticated polishing to improve precision of the first to fourth right angle prisms.
Moreover, in the method for manufacturing a dichroic prism according to the present invention described above, an ultraviolet curable resin, which has the same refractive index as the first to fourth right angle prisms and is cured by being subjected to ultraviolet irradiation, is preferably used for bonding of the first right angle prism and the second right angle prism, for bonding of the third right angle prism and the fourth right angle prism, and for bonding of the first bonded prism and the second bonded prism.
The ultraviolet curable resin is cured when being irradiated with ultraviolet rays. Therefore, for example, in a stage of forming the dichroic prism with the first bonded prism and the second bonded prism, at least one of the first or the second bonded prism is coated with the ultraviolet curable resin, and the first bonded prism and the second bonded prism are temporarily bonded. In this stage, the ultraviolet curable resin is not cured, and therefore the position of the second bonded prism can be adjusted with respect to the position of the first bonded prism. By being irradiated with the ultraviolet ray when the position adjustment is completed, the first bonded prism and the second bonded prism can be bonded an optimal state.
Incidentally, a tenth step of forming the dichroic film for the first color light on the first side face of the first right angle prism may be performed in place of the second step and the third step. An eleventh step of forming the dichroic film for the second color light on any one of the plane, which is formed of the second side face of the first right angle prism and the first side face of the second right angle prism in the first bonded prism, and the plane which is formed of the second side face of the third right angle prism and the first side face of the fourth right angle prism in the second bonded prism, may be performed in place of the seventh step. Moreover, a twelfth step of allowing the first bonded prism and the second bonded prism to contact each other with the dichroic film for the second color light interposed therebetween to constitute the pre-dichroic prism maybe performed in place of the seventh step.
According to yet another aspect of the present invention, a dichroic prism manufacturing device used for the manufacturing method of the dichroic prism described above is provided. The dichroic prism manufacturing device comprises: a holder having a holding portion which holds the first bonded prism; a slide unit provided slidably on the holder, the slide unit being allowed to contact the second bonded prism arranged to constitute the pre-dichroic prism together with the first bonded prism held by the holder, and sliding the second bonded prism with respect to the first bonded prism; a position adjustment unit for sliding the slide unit with respect to the holder, the position adjustment unit being attached to the slide unit; a light source unit for making the light incident on the pre-dichroic prism held by the holding portion; and an image display unit for displaying the image formed of the light from the light source unit, the light being made incident onto the pre-dichroic prism held on the holder and being reflected on the first bonded face and the third bonded face.
With the above-described structure of the dichroic prism manufacturing device, the position of the second bonded prism with respect to the first bonded prism can be adjusted while actually viewing the image.
These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.